Kinetics of the enolisation reaction of 2-acetylfuran, 2-acetyl-N-methylpyrrole and 3-acetyl-N-methylpyrrole

Abstract

Kinetic results are reported on the rates of enolisation of 2-acetylfuran, 2-acetyl-N-methylpyrrole and 3-acetyl-N-methylpyrrole in water, in buffer, in dilute hydrochloric acid solutions, in dilute sodium hydroxide solutions and in the presence of several metal ion salts, as measured by their rates of halogenation. The results have been compared with those of corresponding reactions of 2-acetylthiophene, 3-acetylthiophene and acetophenone. In metal ion promoted reactions the catalytic effects observed are probably due to coordination of the carbonyl oxygen in the reaction transition states, while there is no evidence of participation of the heteroatoms of the aromatic rings in chelation. Metal activating factors are particularly high for the acetylpyrroles where, for example, Cu²⁺ is up to 740 times more effective than the proton in catalysing the enolisation reaction of 3-acetyl-N-methylpyrrole. This is in agreement with the high negative charge density on the oxygen atom recently observed by NMR spectroscopy for the acetylpyrroles as compared to acetylfurans, acetylthiophenes and acetophenone.